Final answer:
To estimate heritability (h2) in twin studies, the first statistical test performed is a hypothesis test, such as a Student's t-test or chi-squared test, to determine variances between monozygotic and dizygotic twins. This involves checking for significant differences in the trait under study, visualized through graphs like box plots.
Step-by-step explanation:
The first graphical/statistical test required to estimate heritability (h2) via twin studies is usually a form of hypothesis testing that compares the similarity between monozygotic twins and dizygotic twins. Typically, the heritability estimate (h2) in twin studies involves comparing the variance in a trait that is attributed to genetic variation versus the variance attributed to environmental factors. The preliminary step involves calculating the correlation coefficient for a trait between twins in each group. Once the correlations are calculated, a hypothesis test is performed.
For instance, in a twin study, the null hypothesis (H0) would state that there is no difference in the trait variance between the two types of twins; the alternative hypothesis (Ha) would be that there is a significant difference in variance, suggesting heritability. The choice of test might involve the Student's t-test or analysis of variance (ANOVA) if comparing means or chi-squared (χ²) tests if analyzing observed versus expected frequencies. The test statistic is calculated from data, and then a graph, such as a set of box plots or a scatter plot, is used to visualize the results.
Finally, evidence supporting the heritability of a trait would reject the null hypothesis in favor of the alternative hypothesis that there are genetic influences on the trait in question, as indicated by a significant difference between the variances of monozygotic and dizygotic twins.